IFCHAN is used to specify the IF channel attached to each baseband channel for VLBA observations using VLBA data aquisition systems. For the VLBA, it should be A or C for 20, 13, or 6cm. For 90, 50, 4, 3, 2, or 1.3cm, it should be B or D. A and B are RCP; C and D are LCP. If DUALX is specified, both B and D are RCP. The new 4-8 GHz system (still called 6cm) can put out signals on all 4 IFs in 2 polarization pairs. The BBCs of the legacy backend can acces any of the 4 IFs. An RDBE can only access 2 (any 2), so the 2 RDBE option is needed to access all 4.
As an alternative to explicit specification of the IF, R or L can be specified. Then if RCHAN and LCHAN are specified in the setup files, IFCHAN='R' will be replaced with RCHAN and IFCHAN='L' will be replaced with LCHAN. If a bad RCHAN or LCHAN value is specified, SCHED will then complain about a bad IFCHAN specification.
1N, 1A, 2N, 2A are allowed values for MkIV telescopes. See BBC and the frequency catalog parameter IFNAME for more details.
A[1-4], B[1-4], C[1-4] and D[1-4] are all possible values for the DBBC. The letter gives the conditioning module (A-D) to use, and the number gives the input (1-4) on that conditioning module to select. See the DBBC section of the manual for more details.
The ``geodetic'' wired VLBA systems (all not controlled by VLBA software) have restrictions on their IF assignments because the IF distributors only provide enough signals from each IF to feed 8 BBCs while these systems typically have 14 BBCs. The systems are wired so that BBCs 1 and 2 can see all 4 IFs, BBCs 3-8 can see A and C, and BBCs 9-14 can see B and D. In addition, the racks can be switched so that they provide one bit samples from all 14 BBCs or 2 bit samples from the first 8. SCHED understands all this and should give the right default assignments. This is one of many reasons why, as noted below, it is best to let SCHED set many parameters including IFCHAN.
It should now be normal not to specify IFCHAN. SCHED will use the requested POL and the frequency catalog to determine the correct IFCHAN. If IFCHAN is specified as ``R'' or ``L'', and RCHAN, LCHAN, and POL are not specified, SCHED will get the obvious polarization channels, although use of POL is recommended.
As of early 2013, the VLA can provide 4 output basebands to the recording system. Each is expected to come from a different IF so they should be assigned to A, and C for RCP and B and D for LCP. Eventually there will probably be an increase in the number and more than one channel will use the same IF.